A storage tank has two inlets. The first alone would fill it in 45 minutes and the second alone in 30 minutes. The first is opened at 10:00 a.m. and the second at 10:15 a.m. At what time is the tank full?

Take the tank as 90 units: the first inlet gives 2 units a minute, the second 3. From 10:00 to 10:15 the first inlet alone puts in 30 units, leaving 60. Both together then give 5 units a minute, so 60 ÷ 5 = 12 more minutes — the tank is full at 10:27 a.m. 10:18 a.m. assumes both inlets ran from 10:00 (90 ÷ 5 = 18 minutes). 10:33 a.m. counts that same 18-minute joint spell from 10:15, forgetting that the first inlet had already delivered a third of the tank. 10:35 a.m. gives the leftover 60 units to the second inlet alone (60 ÷ 3 = 20 minutes), overlooking that the first inlet keeps running.